Dielectric filters are widely used in cellular telephones and other RF devices, and this use has driven demand for further reductions in size and weight accompanied by performance improvements. Dielectric filters can be broadly classified as devices using coaxial resonators and devices using multilayer resonators. Reducing the size of coaxial resonator devices, however, is generally considered to have nearly reached the practical limit, and multilayer filters are therefore preferable for further reducing the size and weight of RF devices.
EP 0 641 035 A2 discloses a laminated dielectric filter comprising a strip line resonator electrode layer forming plural strip line resonators, and a capacity electrode layer, wherein the strip line resonator electrode layer and capacity electrode layer are sandwiched by two shield electrode layers, and the space between the two shield electrode layers are filled with a dielectric, and the thickness between the strip line resonator electrode layer and capacity electrode layer is set thinner than the thickness between the strip line resonator electrode layer and shield electrode layer and the thickness between the capacity electrode layer and shield electrode layer.
The structure of a dual band bandpass filter built using a conventional coaxial filter is described below with reference to FIGS. 30 to 32. FIG. 30 is a perspective view of a conventional coaxial filter comprising as shown dielectric coaxial resonators 30a and 30b, external electrodes 31a and 31b, inside circumferential electrodes 32a and 32b formed inside through-holes in the dielectric coaxial resonators 30a and 30b, connecting metal 33a and 33b, coupling substrate 34, and base 35. This coaxial filter device is described further below as thus comprising a first coaxial filter with a 950-MHz bandpass and a second coaxial filter with a 1.9-GHz bandpass.
FIGS. 31A and 31B are input and output Smith charts for the first and second coaxial filters, respectively, with impedance at 950 MHz indicated at point 22 and impedance at 1.9 GHz indicated at point 23. The characteristics of each single coaxial filter are such that the first coaxial filter has low impedance to the passband of the second coaxial filter and thus shorts, and the second coaxial filter has low impedance to the passband of the first coaxial filter and thus similarly shorts. A dual-band bandpass filter therefore cannot be produced by simply connecting two such coaxial filters together because the filters short at the passband of the other. Phase shifting is therefore required so that impedance to the passband of the other filter is high and an open state is maintained.
FIG. 32 is a circuit diagram of a one-input, one-output dual-band bandpass filter using conventional coaxial filters. Shown in FIG. 32 are a first coaxial filter 35a and second coaxial filter 35b, and first and second phase shifters 36 and 37. The operation of this dual-band bandpass filter is described below.
The impedance of the first coaxial filter 35a to the passband (1.9 GHz) of the second coaxial filter 35b is increased by the matched phase shifter 36. The impedance of the second coaxial filter 35b to the passband (950 MHz) of the first coaxial filter 35a is likewise increased by the matched phase shifter 37. By thus connecting appropriate phase shifters in line with the coaxial filters, the two filters can be connected without affecting the operation of the other filter, and a bandpass filter with two passband can be provided.
As will be obvious from the above description, however, this design necessitates the use of two coaxial filters together with additional phase shift circuit components. Filter size is thus increased, and there is a limit to the size reductions that can be achieved.